In prior art, there are several embodiments of optical voltage sensors integrated into gas-insulated switchgear: U.S. Pat. No. 6,016,053 [1] discloses a voltage transformer formed by an optical voltage sensor incorporated into gas-insulated switchgear, wherein the voltage sensor comprises a Pockels crystal, such as a BGO crystal, with two parallel surfaces, that are perpendicular to the electric field lines and are coated with a transparent, electrically conductive layer. The crystal is rigidly fixated within an insulating tube and with flanges at both ends that form a low-voltage and a high-voltage end of the transformer. The voltage transformer is rigidly fixated by a carrying arm within the switchgear at the low-voltage end; at the high-voltage end, the voltage transformer is supported in a shielding cover by rubber elastic foam rings and a bearing ring as well as a contact spring. The voltage transformer further comprises a gas/molecular filter to remove reactive decomposition products of the insulation gas and moisture from the gas compartment.
DE 196 04 908 A1 [2] discloses a voltage measurement device incorporated into gas-insulated switchgear. The device comprises a cylinder-shaped quartz crystal between high voltage and ground potential. An optical fiber is wrapped onto the cylinder surface. The periodic piezo-electric deformation of the crystal as a result of an applied alternating current (AC) voltage produces a differential optical phase shift of light waves propagating in the fiber which serves as a measure for the voltage. The quartz crystal is equipped with metallic fittings at both ends. The fittings are in electric contact with field steering electrodes by means of contact springs. Vibration damping material between the fittings and electrodes protects the crystal against mechanical vibrations and shock. An insulating pipe between high voltage and ground electrodes encloses the crystal. The hollow volume inside the pipe is in gas exchange with the switchgear gas volume via a cartridge. The cartridge contains an absorber material that prevents reactive decomposition products of the insulation gas produced by electric arcing from reaching the crystal.
WO 2009/080109 A1 [3] discloses an optical voltage sensor incorporated into gas-insulated switchgear. An electro-optic rod or fiber resides within a bore of a partition insulator which extends radially from the bus bar on high voltage to the metal enclosure on ground potential. The bore is filled with oil or resin. The electro-optic fiber or rod may be inserted, in addition, into an oil-filled capillary.
U.S. Pat. No. 5,136,236 [4] and EP 1 710 589 (A1) [5] describe further embodiments of optical voltage sensors incorporated into gas-insulated switchgear, wherein the voltage measurement is reduced to a measurement of local electrical field.
K. Kurosawa et al., “Development of an optical instrument transformer for DC voltage measurement,” IEEE Transactions on Power Delivery, vol. 8, pp. 1721-1726, 1993 [6] and F. Cecelja, et al., “Validation of electro-optic sensors for measurement of DC fields in the presence of space charge,”Measurement, vol. 40, pp. 450-458, 2007 [7] disclose optical voltage sensors for direct current (DC) voltage. Both embodiments employ an electro-optic crystal that exhibits linear birefringence in the presence of electrical fields transverse to the direction of light propagation.
U.S. Pat. No. 5,715,058 [8] lists different electro-optic crystal classes and crystal orientations together with corresponding materials that enable line integration of the electrical field without exhibiting birefringence at zero electrical field. In addition, a sensor configuration is disclosed with the electro-optic crystal attached between two glass plates that act as holding elements and as substrates for the layer electrodes formed by a transparent or reflective electrically conducting film.
In the context of optical voltage sensors for alternating current (AC) voltages, field steering and shielding of unwanted stray fields have been considered:
WO 00/34793 [9] and Chavez et al., IEEE Transactions on Power Delivery 17, 362 (2002) [10], disclose optical voltage sensors comprising one or more local electrical field sensor(s) distributed along a path from ground to high voltage inside an insulating tube or section; insensitivity to electrical stray fields is achieved by a specified permittivity and geometry of the insulating section.
WO 2011/154408 A1 [11] discloses an optical high voltage sensor for outdoor use comprising a cylinder-shaped electro-optic or piezo-electric crystal in the longitudinal bore of an epoxy insulator pole. The insulator pole contains two sets of embedded electrode foils in a concentric and staggered arrangement in order to achieve an advantageous electric field distribution. The remaining hollow volume of the bore is filled with filler material. In a preferred embodiment, an electro-optical crystal is equipped with electrically conducting electrodes to control the field distribution near the crystal ends. The electrodes are attached to the sensor crystal in a flexible manner, i.e. by means of rubber o-rings or silicone. Alternatively, the contacting electrodes can be made from electrically conducting rubber or elastomers. The electrodes are in electric contact with the ground and high voltage potentials by electric wires. Also, the crystal facets may be coated with optically transparent conductive films which are again electrically contacted by wires. This design is disadvantageous for measurement of DC voltages as space charges may accumulate in the insulating material with increased risk of dielectric breakdown.
Schemes for interrogation of the electro-optic phase shift induced by the electro-optic effect or Pockels effect in an electro-optic crystal are e.g. disclosed in [1], [2], [11], and WO 2008/077255 A1 [12]. These embodiments comprise interrogation schemes with the electro-optic crystal integrated into the optical path of the interrogation system in transmission or in reflection as well as polarimetric schemes, wherein the phase shift is converted into a change of light powers by means of passive optical elements such as retarders and polarizers, and interferometric schemes, wherein the phase shift is modulated by an optic modulator.
Further Embodiments of Optical Voltage Sensors:
EP 1 462 810 A1 [13] discloses an optical voltage sensor, wherein the sensing element is formed as a stack of alternating layers of quartz glass and electro-optic crystals. The contact electrodes are attached to both ends of this stack so that voltage to be measured drops over the entire stack.
EP 0 907 084 B1 [14] discloses an optical voltage sensor consisting of a series of cylinder-shaped piezo-electric quartz sensing elements with an attached optical fiber, separated by metal tubes, and arranged in a hollow-core high voltage insulator. The ends of each quartz element are equipped with an inner electrode of appropriate thermal expansion attached to the crystal by an electrically conducting glue, a field steering outer electrode, and an intermediate metal plate. The sensing element with the attached electrodes is flexibly connected to the neighboring metal tubes by spring elements. The whole assembly is embedded into polyurethane foam.
Designs of prior art for integration of voltage sensors in gas-insulated switchgear, e.g. [1], [2], and [3], have in common that the voltage sensing element resides in a tube-shaped insulator which is disadvantageous for direct current (DC) voltage sensing, as space charges can accumulate in that insulator and thus can impact the dielectric strength of the arrangement. Further embodiments of prior art [4], [5] rely only on local magnetic field measurement and are hence prone to influences from stray fields, e.g. stemming from space charges, so that they are, in particular, is advantageous for measurement of DC voltages. Voltage sensors of prior art especially designed for measurement of DC voltages [6], [7] show, due to a crystal geometry with the voltage applied transverse to the optical path, a sensitivity to space charge effects within the crystal, which is as well disadvantageous for DC voltage measurement, as it results in signal drifts over time.